The Internet and the so-called World Wide Web (the “WWW”) have become ubiquitous. Thousands or even tens of thousands of so-called content providers (publishers) now use the Internet (and, particularly, the WWW) to provide all sorts of content to tens or even hundreds of thousands of clients all over the world.
In order to offload the job of serving some or all of their content, many content providers now subscribe to so-called content delivery networks (CDNs). Using a CDN, some (or all) of a content provider's content can be served to clients from the CDN (i.e., from one or more servers in the CDN) instead of from the content provider's server(s). In a caching CDN, content that is served may also be cached on some or all of the CDN servers, either before being served or in response to specific requests for that content.
Certain publishers have large content libraries in which only a small proportion of the content (the so-called “short head”) is popular enough to benefit from serving through a caching CDN, while the majority of the content (the so-called “long tail”) is accessed only occasionally and not generally worth caching. This situation would be typical for a content publisher with a very large music or video library. Some music content—the popular content—may be regularly requested, whereas other music—the not popular (also referred to as unpopular) content—may be seldom if ever requested.